1. Field of the Invention
This invention relates to an ignition system for an internal combustion engine, and more particularly to an ignition system for an internal combustion engine which is adapted to control an ignition position of the engine depending on a rotational speed of the engine.
2. Description of the Prior Art
In order to prevent overspeed of an internal combustion engine, it is often required to delay an ignition position of the engine when a rotational speed of the engine reaches a high speed region. Also, in a two-cycle engine, in order to increase an output of the engine when the engine is rotating at a high speed, it is often carried out to delay its ignition position. For these purposes, it is required to construct an ignition system so that it may exhibit characteristics of causing the ignition position to be delayed when a rotational speed of the engine is in a low or middle rotational speed region and to be advanced when it reaches a high rotational speed region.
Recently, it is desired that an ignition system for an internal combustion engine controls an ignition position of the engine as accurately as possible. In order to meet the requirement, an ignition system of the electronic control type has been extensively used which is adapted to calculate an ignition position of an engine corresponding to a rotational speed of the engine by electronic operation. Such an ignition system is disclosed in Japanese Patent Application Laying-Open Publication No. 25968/1986, the disclosure of which is incorporated by reference. The conventional ignition system includes a signal coil for generating a maximum advanced position indicating signal and a minimum advanced position indicating signal, an integrating circuit controlled by the signals generated from the signal coil, and a comparator for comparing a integral signal supplied from the integrating circuit with a reference signal. Comparison between the integral signal and the reference signal leads to advance characteristics or delay characteristics. Accordingly, the conventional system exhibits desired advance characteristics and delay characteristics by adjusting an integration constant of the intregrating circuit.
However, the conventional ignition system of the electronic control type is complicated in its circuit structure and increased in manufacturing costs because of requiring the integrating circuit for operation of the ignition position and a control circuit for controlling the integrating circuit. Accordingly, the ignition system is not applied to an engine of which costs are desired to be reduced or restrained. Also, it is required that the signal coil generates signals at the maximum advanced position and minimum advanced position, therefore, a coil which is arranged in a usual magneto mounted at an engine cannot be used as the signal coil. This needs to arrange a separate signal generator, resulting in a further increase in costs of the ignition system.